Conventionally, active ray-curable compositions, which are cured with active energy radiation such as UV radiation or electron beams, have been put into practice use in a wide variety of end use applications such as paints, adhesives, printing inks, printed circuit boards, and electrically insulating materials for plastics, paper, woodworks, and inorganic materials. Over recent years, in methods for forming an image utilizing active ray-curable ink, further enhanced adhesion to a recording medium and flexibility improvement of a cured film after curing have been demanded.
Especially in cases when image formation is carried out on a fibrous material such as a textile using active ray-curable ink, there have been noted such significant problems that feel of quality of the textile is impaired due to insufficient flexibility of a cured film, and crumpling of the image-formed textile results in peeling of the cured film.
On the other hand, for an ink-jet recording system utilizing a polymerizable composition, there is UV curable ink-jet ink which is cured with UV radiation. An ink-jet system utilizing this UV curable ink-jet ink is receiving much attention due to quick-drying properties and superior recordability to recording media with no ink absorbability. Accordingly, various types of UV curable ink-jet inks have been developed (for example, please refer to Patent Documents 1 and 2).
However, it is disadvantageous that since a formed cured film exhibits poor flexibility, the cured film tends to crack due to no conformity to a recording medium. Therefore, there has been produced the problem that printing can be performed on only limited types of recording media.
In contract, disclosed is a method wherein a plasticizer is added to an ink-jet ink for ink-jet printing to provide plasticity (for example, please refer to Patent Document 3).
However, the method described in Patent Document 3 relates to a melt-type ink-jet ink, and no UV curable ink-jet ink is described at all.
Further, mere addition of a plasticizer produces the problems that insufficient curability is expressed and a cured film (printed matter) is vulnerable to damage.
Still further, when a UV curable ink-jet ink containing a pigment, a photoinitiator, and a polymerizable compound is used, it is disadvantageous that ejection tends to be unstable depending on the operational ambience. Especially when a flexibility enhancer such as a plasticizer is added as described above, such characteristics are markedly exhibited.
On the other hand, there is noted such a problem that an ink employing a cationically polymerizable compound is sensitive to moisture (humidity) of a molecular level (for example, please refer to Patent Documents 4-7).
As described in Patent Documents 4-7, there is widely known a technology wherein a polycyclic aromatic compound, having, as substituents, a hydroxyl group and at least one of an aralkyloxy group and an alkoxy group which may be substituted, is used as a sensitizer to enhance curability.
However, when a polycyclic aromatic compound is used, there has been noted such a problem that precipitation of the above-described photoinitiator and dispersant is induced, whereby ejection stability after cooled storage and/or after high temperature storage is further deteriorated.    Patent Document 1: Unexamined Japanese Patent Application Publication No. (hereinafter referred to as JP-A) 6-200204    Patent Document 2: Japanese Translation of PCT International Application Publication No. 2000-504778    Patent Document 3: JP-A 8-3493    Patent Document 4: JP-A 2002-188025    Patent Document 5: JP-A 2002-317139    Patent Document 6: JP-A 2005-113043    Patent Document 7: JP-A 2006-37021